An Optical Survey of Outlying Ejecta in Cassiopeia A: Evidence for a Turbulent, Asymmetric Explosion
A deep optical survey of the Cassiopeia A supernova remnant has revealed dozens of new emission-line ejecta knots out beyond the remnant's bright nebular shell. Most of the newly detected knots exhibit a 4500-7500 Å spectrum dominated by [N II] λλ6548,6583 line emissions. After accounting for possible decelerations, the estimated space velocities for about four dozen of these [N II] knots suggest a nearly isotropic ~=10,000 km s-1 ejection velocity. However, a small group along the southwestern limb show significantly higher velocities of up to 12,000 km s-1. Over 20 outlying O+S emission knots were also discovered, mostly along the remnant's western limb. These knots have optical spectral properties like those seen in the main shell's metal-rich ``fast-moving knots'' but with much higher estimated space velocities of between 7600 and 12,600 km s-1. Discovery of these knots means that the remnant's highest-velocity, O+S debris are not confined to just the remnant's northeast ``jet.'' [S II] λλ6716,6731 emissions dominate the spectra of these knots above an expansion velocity of 11,000 km s-1. A few ``mixed emission knots,'' which show both strong nitrogen and sulfur line emissions, were also detected along the remnant's western rim. The properties of these outlying debris knots suggest a turbulent supernova expansion in which the innermost S-rich layers were ejected up through overlying material in certain regions, attaining final outward velocities greater than the star's N and He-rich surface layers. The detection of such high-velocity, sulfur-rich ejecta only along the remnant's northeast and southwest limbs further suggests an asymmetric expansion, possibly bipolar. A turbulent expansion may help explain the creation of the observed mixed emission knots. It is unclear, however, if mixed knots represent truly microscopically mixed debris or are simply small, comoving clusters of chemically distinct ejecta.